1. Field of the Invention
The present invention relates to a DC-to-DC converter circuit, and more particularly to a double forward DC-to-DC converter circuit with soft-PWM switching which includes a main switch, an auxiliary switch, rectifier diodes and a transformer for transferring energy from a primary winding of the transformer to a secondary winding when the main switch is under the control of a PWM circuit. The DC-to-DC converter circuit includes circuitry for automatically resetting the energy stored in the transformer winding when the main switch is turned off, without the possibility of being locked up as well as circuitry for soft switching of the rectifier diodes to eliminate reverse recovery losses associated with the rectifier diodes connected to the transformer secondary winding. The main and auxiliary switches are both turned on at zero voltage while snubber capacitors are provided to minimize turn-off losses.
2. Description of the Prior Art
DC-to-DC converters are used to convert an unregulated source of DC power into a source of constant voltage for use in various applications. Such DC converters normally include a transformer having primary and secondary windings. A switch, for example, a solid state switch, is connected to the primary winding in order to control energy transfer from the primary to secondary winding. In PWM-controlled converters, the switch is normally under the control of a pulse width modulator (PWM) circuit which varies the duty cycle, defined as switch on time over switching period.
With the increase of switching frequency to reduce the size and weight, as constantly required in aerospace and military applications, the switching losses increase rapidly. To cope with this, DC-to-DC power converter designers employ various schemes to eliminate or to minimize losses associated with switching of the solid state switches. These schemes are generally referred to as various techniques of soft switching. Among all kinds of soft switching schemes, the most promising ones are those which have zero-current or zero voltage switching at turn on and turn-off transitions, while keeping voltage and current stresses similar those in a PWM (hard-switched) converter. This is so because stress levels for switch voltage and current similar to those in a PWM (hard-switched) converters represent the best possible efficiency in power transfer. This group of soft switching can be called soft-PWM switching.
In recent developments the popular phase-shift scheme for soft switching comes close to realize soft-PWM waveforms. However, its applications are limited to double-ended converters, such as half-bridge and full-bridge converters. Most recent developments of soft switching schemes for single-ended converters, such as forward and flyback, utilize a mechanism known as the current mirror to reset the transformer automatically. Examples of such circuitry are disclosed in U.S. Pat. Nos.: 4,441,146; 4,809,148; 4,959,764 and 5,126,931. Examples of such circuitry is also disclosed in the following publications: Constant Frequency ZVS Converter with Integrated Magnetics, by J. A. Bassett, publication no. 0-7803-0485-3/92, 1992, IEEE, pp. 709-716; High Frequency, Soft Transitions Converter, by I. D. Jitaru, IEEE publications no. 0-7803-0982-0/93, 1993, IEEE, pp. 880-887; Switched Snubber for High Frequency Switching, by K. Harada and H. Sakamoto, IEEE publication no. CH2873-8/90/00000181, 1990, IEEE, pp. 181-187 and Design Techniques for Transformer Active Resets Circuits at High Frequencies and Power Levels, by B. Carsten, HFPC May 1990 proceedings, pp. 235-245. Such circuits typically include a second solid state switch and a capacitor for transferring the magnetizing current stored in the transformer winding when the main solid state switch is opened, recycling energy back to the DC voltage source connected to the transformer primary winding. Such circuitry causes the main and auxiliary switches to be turned on at zero voltage and includes a lossless snubber, normally a capacitor connected in parallel across the switch. The snubber is used to minimize the voltage stress and losses when the switch is turned off. In addition, such circuits are configured to discharge the snubber capacitor prior to closing of the switch to eliminate the turn on loss and to minimize the voltage stress across the switch when the switch is turned on. Examples of such circuitry are disclosed in U.S. Pat. Nos. 4,959,764, 5,126,931 and 5,231,563.
Note that the technique of the current mirror, used in the cited patents, may cause the circuit to operate in a mode locked-up to a subharmonic frequency. The locked-up mode of operation forces the output voltage to go out of regulation and may potentially damage the switching elements. An additional protection circuit is used to guarantee safe operation of the converter.
As disclosed in U.S. Pat. Nos: 4,809,148; 4,441,146; 4,959,764 and 5,126,931, DC-to-DC converters normally include one or more rectifier diodes, connected to the transformer secondary winding. Typically one rectifier diode is connected in parallel across the secondary winding of the transformer while a second rectifier diode is connected in series with the load. In any switching converters, the rectifier diodes are subject to what is known as reverse recovery losses which result when the biasing across the diode quickly reverses. Various techniques are known for minimizing the reverse recovery losses of the rectifier diodes, for example, as disclosed in High Frequency, Soft Transitions Converter, by I. D. Jitaru, publication no. 0-7803-0982-0/93, 1993 IEEE, pp. 880-887 (and in U.S. Pat. No. 5,434,768). However, the technology, as the author stated, "does not eliminate totally the reverse recovery loss of the diodes."
In short, existing converters circuits, while implementing effectively soft switching, suffer from two major defects: the dangerous locked-up mode, which may destroy parts, and the excessive reverse recovery losses of diodes, which compromises power conversion efficiency.